The present invention relates to a locking mechanism for an injection mold and in particular to a locking mechanism for holding a core rod in place when injection molding a plastic preform thereon for subsequent blow molding to form a container.
Most containers blow molded from polyethylene terephthalate (PET) are made by first injection molding a preform over the end of a core rod. After injection molding the preform, two injection mold halves separate laterally from the core rod and the core rod carrying the preform is transferred to a blow mold where the preform is then blow molded into conformity with the blow mold cavity forming the finished product.
The preform is generally in the shape of an elongated hollow body open at one end and closed at the other end. The preform is molded by first closing two mold halves forming a cavity open at one end and placing a core rod in the cavity through the open end. A hollow space is formed between the core rod and the mold cavity corresponding to the desired shape of the preform. The plastic resin is typically injected into this space at the center of the closed end of the preform.
The neck finish at the open end of the molded container is completely formed during the injection molding of the preform. This neck finish, which typically contains the threads or other means for securing a closure to the container, is not later reshaped during the blow mold stage. As a result, the diameter of the preform open end is the same as the diameter of the finished container open end.
During the injection molding process, the resin pressure acts on the core rod in a direction to force the core rod from the mold cavity. As the diameter of the core rod increases, so does the force acting to move the core rod from the mold cavity. To properly mold a preform, it is necessary that the core rod remain stationary during injection molding.
Accordingly, it is an object of the present invention to provide a locking mechanism to prevent withdrawal of the core rod caused by the injection pressures.
The locking mechanism of the present invention provides a pair of cam shafts with one cam shaft carried by each mold half. The cam shafts extend parallel to the mating face of the two mold halves. When the mold halves close, eccentric cams carried by the cam shafts are positioned within recesses formed in the core rod above the open end of the preform. The cam shafts are then rotated to move the eccentric cams into a locking position within the recesses to prevent withdrawal of the core rod from the mold cavity. The cams and core rod recesses are configured such that the forces applied to the cams by the core rod act substantially radially through the cam shaft's rotational axes so as to create little or no rotational moment in the cam shafts tending to disengage the cams. Hydraulic cylinders are used to rotate the cam shafts into the lock position and only a minimal force is required to maintain the cam shafts in the lock positions during the injection molding process.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.